Method of refining vegetable and animal oils



Patented May 18, 1954 METHOD OF REFINING VEGETABLE AND ANIMAL OILS Ivar Axel Afzelius, Bromma, and Hans Olof Lindgren, Smedslatten, Sweden, assignors to Aktiebolaget Separator, Stockholm, Sweden, a corporation of Sweden No Drawing. Application March 1, 1950,

. Serial No. 147,148

11 Claims. 1

Fatty oils, particularly linseed oil, have to a great extent been refined with lye in a continuous process. Heretofore, the lye in great excess has been continuously mixed by vigorous stirring into flowing oil, in the form of fine drops, with the double purpose of neutralizing free fatty acid and precipitating slime and coloring matter in the oil. As a rule, the lye was mixed into cold oil, and the mixture heated during stirring to so high a temperature that the soap was flaked out. The stirring was interrupted only when the pigment and the slimy substances had been completely precipitated. The oil was then freed from soap, pigment and slimy substances by centrifuging.

The linseed oil has in this way been refined with smaller losses than was possible with the older periodical procedure, by which lye was mixed batch-wise in great quantities of oil, the soap, pigment and slimy substances being eliminated by settling. In the periodical process, the mixture was also heated while stirring in order to precipitate the soap in the form of flakes.

Refining of linseed oil, for example, also caused great oil losses in the continuous method previously described. The soap formed upon neutralization of the fatty acids was split up by the vigorous stirring. An emulsion was then formed which was not broken completely by heating. In

spite of centrifuging, the soap therefore contained considerable quantities of oil.

Oil emulsified in the soap does not represent the entire oil loss during refining. Heavily colored oils which are to be decolored in a satisfactory manner will need much more lye than is combined with the free fatty acids in the oil. Part of the excess lye also splits up some neutral oil. This reaction is accelerated by the vigorous stirring, particularly during heating.

In order to reduce the losses of neutral oil, it has also been proposed to proceed as follows: Lye is injected into an oil current in such a way that the lye is dispersed in the oil as fine drops. The mixture then flows through a space without substantial further mixing, whereupon the soap agglomerates and pigment and slimy substances are precipitated. The soap resulting from the subsequent centrifuging of the mixture has a lower content of neutral oil than that obtained in the method described previously. This method may be used to advantage, e. g. for cocoa-nut oil which need not be decolored, but not for linseed oil. In the latter the pigment and slimy substances are not sufficiently precipitated.

The following is another process which gives a satisfactory result for linseed oil as regards yield and color. The refining takes place in two stages. In the first stage, free fatty acids are neutralized with soda. The soap formed is removed by centrifuging. Before this, great quantities of water must be evaporated in order to liberate the soap from carbon dioxide bubbles adhering to it. Part of the carbon dioxide formed at the neutralizing otherwise adheres to the soap, the specific gravity of which is thereby reduced so that it cannot be eliminated by centrifuging.

In the second stage, pigment and slimy substances are precipitated from the oil arriving from the first stage, by lye. The latter is mixed with the oil during vigorous stirring which is interrupted only when slime and coloring substances have been entirely precipitated. The oil is also centrifuged in the second stage.

As pointed out previously, this mode of operating gives a high quality oil with small refining losses. There are, however, great drawbacks in this connection. The neutralizing requires a great excess of soda, which as a rule is more expensive than lye. Besides, the plant is expensive and complicated by the apparatus for expelling the carbon dioxide. The method can therefore be applied only to big plants where mainly linseed oil is treated and where there is expert supervision.

The present invention resides in an improved method of refining fatty oils with lye in two stages, with different modes of working in both stages. In the first stage, strong lye is rapidly mixed with the oil in the form of fine drops, but in insufficient quantities to completely neutralize all of the fatty acids and acid soaps in the oil,

in order to neutralize a substantial part of the free fatty acids. This partial neutralization; and preferably also agglomeration, is completed without substantial atomization of the drops formed. when mixing the lye with the oil, whereupon the mixture is freed from soap and acid soap by centrifugal separation. In the second stage, the remainder of the oil from the first stage is mixed with a further quantity of strong lye for the purpose of neutralizing remaining acid soaps and fatty acids and precipitating pigment and coloring matter, whereupon the mixture is again subjected to centrifugal separation to remove the oil from the soap formed and from the pigment and slimy matter.

It is important to reduce the oil losses owing to the formation of emulsion in the first stage. To that end the lye is rapidly and intimately mixed with the oil, although too small drops, smaller than 1, ,c should be avoided. Neutralizing in combination with ole-coloring and de-sliming takes place without any further splitting up of the lye drops formed when mixing the lye. Mechanical stirring should therefore be avoided and the lye globules should be kept suspended in the oil and constantly brought into contact with new oil molecules by turbulence brought about by a convenient rate of flow. If a lye globule hits an oil molecule with free fatty acid, this is neutralized. The soap then formed travels into the globule, whose lye content is gradually reduced. The formation of an emulsion and therefore the refining losses in the first stage depend to a high degree on the quantity of finely dispersed soap. It is thus of the greatest importance that lye travels outward by diffusion into the outer layer of the globules and that the neutralizing takes place without splitting up globules whose outer layers for the most part consist of soap.

If emulsification or an excessively fine dispersion of the soap globules is avoided, these are easily separated by centrifuging. In order to obtain a good result, fine soap particles should not be allowed to split on in the centrifuge. The mixture of oil and soap should be introduced into and fiow through the rapidly rotating separating space without any vigorous shocking action and turbulence. As the risk of splitting up is greatest in the inlet space of the centrifugal bowl, we prefer to use hermetic separators in which the liquid is led by over-pressure into the central part of the separator bowl, into a space which is always filled with liquid. From the separator bowl, oil and soap can be discharged either through outlets which are tightly connected to fixed pipes by packings, through paring devices, or through orifices in the bowl hood. In both the last-mentioned alternatives, the counter-pressures in the paring devices or the distance of the orifices from the axis of rotation should be adapted so that the inlet space is always filled with liquid.

The separation is made easier if it is effected at an increased temperature. If the lye is mixed into cold oil, the mixture must be heated during or after neutralizing. As vigorous stirring must be avoided, the heat transmission between the oil and the heating surfaces will be slow. We therefore prefer to mix the lye into the oil when it is at a temperature somewhat higher than the separating temperature. The neutralizing action is thereby accelerated by increased rate of diffusion.

In the above it has been pointed out that a substantial part of the free fatty acids of the oil. are neutralized in the first stage. The applicants have found that incomplete neutralization in the first stage brings about certain advantages compensating for the drawbacks of an increased soap-quantity in the second stage and, consequently, a reduced yield of neutral oil. If a smaller quantity of lye is added in the first stage than that theoretically corresponding to complete neutralizing, this will bind more fatty acid than the amount calculated from the chemical composition of the fatty acids. If there is a deficiency of lye, part of the fatty acids are likely to form acid soaps, whereas another part is adsorbed by the soap. Through this the consumption of lye is reduced and there is also a reduced consumption of acid during the splitting up of the soap. This mode of operation has been found particularly convenient with a high free fatty acid content, that is, when up to 20% of the free fatty acids have been left in the oil from the first stage.

The oil obtained in the first stage which, as stated above, contains some remaining free fatty acids, is treated in the second stage with lye in a quantity depending on the nature of the oil and on the desired degree of de-colorization. The lye should be intimately mixed with the oil and stirred vigorously as this seems to be necessary for the decoloring.

The reactions in the second stage must often take place at an increased temperature in spite of the disadvantage with increased saponification of neutral fat which is enhanced by vigorous stirring. This process, undesirable in itself, has, however, a favorable efiect on the de-coloring, because finely divided soap is then formed which has a vigorously adhesive effect on coloring substances dissolved in the oil. When refining oils which are difiicult to decolor, it is therefore advantageous to neutralize the oil incompletely in the first stage. During final neutralizing in the second stage, the resulting soap has a strong adhesive effect on the pigment substances of the oil owing to its fine dispersion. In order to bring about a complete de-coloring, it is therefore unnecessary to have as large soap quantities formed by saponification of neutral fat as in the decoloring of entirely neutralized oil.

Before the soap is separated from the oil in the second stage through centrifuging, it is advantageous to agglomerate it along with precipitated pigment and slimy substances. This agglomeration takes place in a space through which oil flows without mechanical stirring and can be accelerated by a weak turbulence in the liquid current. The separator types best used for separating are those described for the first stage.

In spite of the agglomeration, the soap from the second stage contains more emulsified neutral oil than the soap from the first stage. In this respect the former is more closely comparable to the soap from the continuous single step method initially described. When the neutralizing of the oil in the first stage is incomplete, the main part of the fatty acids in this stage are removed as soap with a low content of neutral oil. The refining can therefore be done with smaller oil losses than when neutralizing and de-coloring in one stage with the emulsifying treatment, which is necessary for complete de-coloring. In the method according to the invention, this treatment afiects only a small quantity of soap and with this a smaller quantity of neutral fat is removed than when refining in one stage, even if the content of neutral oil in the soap is equal in both products.

The invention also includes modifications of the above procedure, which can be used to advantage in the refining both of oils containing a high proportion of free fatty acids (more than 8% by weight) and of oils with a low proportion of fatty acids (preferably less than 5% by weight).

An installation or a part of a large installation for continuous two-stage refining comprises a separator in each stage. Under normal conditions of work, the separators in both stages should have approximately the same separating efliciency, that is, the same separating ability.

It is to be noted that the separators should be capable of removing from the oil substances emulgated therein, as soap, coloring and slimy matter, and also of de-oiling these substances as completely as possible. In order to fulfil these two requirements, the soap-oil mixture must be.

fed into the separating chamber at a suitable distance from the axis of rotation. In the space inside the inlet zone the oil is freed from emulgated substances; in the space outside this zone the substances are de-oiled. The inlet zone should be arranged at such a distance from the axis of rotation that, under normal conditions of work, the outer space de-oils to the desired I degree those emulgated substances which are separated from the oil in the inner space when the machine is operating at its optimum capac-- ity. If the outer space is made unnecessarily large, the efficiency of the separator is impaired, because the separating eificiency of the outer space is at the same time reduced. It is true that the inner space is then capable of purifying a larger quantity of oil, but the outer space cannot de-oil the separated soap to the desired extension. The reduced separating ability of the outer space thus makes it necessary to reduce the throughput in order to attain the desired deoiling. On the other hand, when the inlet zone is located too close to the rotation axis, the separating efficiency of the inner space will determine the capacity.

We have found that, when separators having the same separating efficiency are used in both stages, oils with different contents of fatty acids should be treated differently in order to allow the installation to work at its optimum capacity.

.In the case of a low content of free fatty acid, particularly below 5%, we have found it of advantage to neutralize the free fatty acids almost completely in the first stage. Optimum capacity and a favorable yield have been achieved when the oil leaving the first stage contains less than 0.3% free fatty acids. The soap formed from this fatty acid during the second stage of neutralization is generally sufiicient to adsorb the coloring matter which may be removed by refining with lye.

In the first stage, the product thus separated and de-oiled consists largely of soap. In the second stage, the emulsion treated in the separator consists of soap and a free finely-flocculated precipitation of slimy and coloring substances as well as free lye, being quantitatively less than the corresponding emulsion in the first stage but more difiicult to de-oil than the latter.

If the same process is used in the neutralization of oils having a very high percentage of free fatty acids, the throughput of the first stage separator must be considerably reduced as compared with that usable in the treatment of oils with low content of fatty acids. In the first instance, the separator must de-oil larger quantities of soap than in the latter, but there would be no risk of a noticeable increase in the load on the second stage separator when treating oils with a high content of fatty acids. When the throughput of the first stage separator is reduced to an extent which insures the desired de-oiling of the soap, the separating efiiciency in the second stage will be incompletely utilized.

The effective capacity of the installation will be increased when part of the separation is transferred from the first to the second stage. This may be achieved by reducing the amount of soap in the first stage by neutralizing the fatty acids less completely in this stage. The degree of neutralization is chiefly determined by the content of free fatty acids but also by the amount of precipitable slime and pigment aand their distribution over the two stages. For this reason it is impossible to state in the form of a formula the degree of neutralization as a function of the proportion of free fatty acids in the oil. The general rule is that an increased content of fatty acid in the crude oil entails an increased content of fatty acid in the oil from the first stage.

We have found that when the content of fatty acid in the crude oil is over 8%, a quantity of lye corresponding to a neutralization degree of less than 0.75 should be used in the first stage, that is, the oil from the first stage should contain not less than 25% of the quantity of fatty acid in the crude oil. The above figures are based on the assumption that neutral soaps are produced in the first stage. When the neutralization is incomplete, acid soaps are formed, and the amount of free fatty acid in the oil from the first stage will actually be less than that stated above.

The following degrees of neutralization have been used to advantage. When refining second quality olive oil having a percentage of free fatty acid of 22.4, an amount of lye calculated to give an oil containing 12% free fatty acids should be added in the first stage. By analysis, 7.5% was found. With this oil, the calculated degree of neutralization was 0.46 and the actual 0.67. With palm nut oil having 15% free fatty acids, the calculated degree of neutralization was 0.40 and the actual 0.50. When refining soya oil having 8% free fatty acids, the corresponding figures were 0.75 and 0.80.

When processing oils which are difficult to refine and particularly to de-color, it was heretofore necessary to use a complicated process involving neutralizing with soda in a first stage and de-coloring with lye in a second stage. Installations for this purpose are, however, very expensive. The invention makes it possible to achieve in a simple and inexpensive manner, with oils difficult to refine, the same qualitative and nearly the same quantitative result as in the two-stage process previously mentioned. The invention also enables small refineries which cannot afford to install and operate a large soda-lyeinstallation to work as economically as a large refinery having the above-mentioned expensive equipment.

We claim:'

1. A method of continuously refining fatty oils in two stages with lye as a neutralizing and decoloring agent in both stages, which comprises, in the first stage, mixing strong lye rapidly with the oil in the form of fine drops but in insuiiicient quantity to completely neutralize all of the fatty acids and acid soaps in the oil, thereby neutralizing a substantial part of the free fatty acids in the oil, the partial neutralizing being completed While fiowing the mixture under conditions of mild. turbulence, centrifuging the mixture in a centrifugal chamber to free the oil from soap and. acid soaps, and, in the second stage, mixing the remaining oil from the first stage with a further quantity of strong lye to neutralize residual fatty acids and acid soaps and precipitate pigment and slimy matter, and recentrifuging said remaining oil to separate the resulting soap, pigment and slimy matter therefrom.

2. A method according to claim 1, comprising also the step of agglomerating, in said first stage, the products of said partial neutralization while maintaining said drops against substantial atomization.

3. A method according to claim 1, comprisingalso the step of heating the oil prior to said mixing in the first stage.

4. A method according to claim 1, comprising also the step of heating the oil prior to said mixing in the first stage, to a temperature higher than the temperature of the mixture at said first centrifuging.

5. A method according to claim 1, comprising also the step of heating the mixture of oil and lye before said first centrifuging.

6. A method according to claim 1, in which the mixture, in the first stage, is fed to said chamber in the absence of substantial impact.

'7. A method according to claim 1, in which the lye and oil are mixed in the second stage with a strong mixing action.

8. A method according to claim 1, comprising also the step of subjecting the mixture in the second stage to mechanical stirring prior to said recentrifuging.

9. A method according to claim 1, in which the degree of partial neutralization in the first stage is so controlled as to completely utilize the separating eificiency in both of said centrifugings.

10. A method according to claim 1, in which, when the content of free fatty acids in the oil at the start of the first stage is higher than 8%, the degree of neutralization in the first stage is less than complete.

11. A method according to claim 1, in which, when the content of free fatty acids in the oil at the start of the first stage is less than 5%, the lye in the first stage is mixed in a quantity such that the oil contains less than .3% free fatty acids at completion of the first stage.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,619,486 Resines Mar. 1, 1927 1,928,613 Barrdas Oct. 3, 1933 2,100,275 Clayton et al Nov. 23, 1937 2,230,796 Froding Feb. 4, 1941 OTHER REFERENCES BaileyIndustrial Oil and Fat Productspage 505, Interscience Publishers Inc., New York, N. Y., 1945. 

1. A METHOD OF CONTINUOUSLY REFINING FATTY OILS IN TWO STAGES WITH LYE AS A NEUTRALIZING AND DECOLORING AGENT IN BOTH STAGES, WHICH COMPRISES, IN THE FIRST STAGE, MIXING STRONG LYE RAPADLY WITH THE OIL IN THE FORM OF FINE DROPS BUT IN INSUFFICIENT QUANTITY TO COMPLETELY NEUTRALIZE ALL OF THE FATTY ACIDS AND ACID SOAPS IN THE OIL, THEREBY NEUTRALIZING A SUBSTANTIAL PART OF THE FREE FATTY ACIDS IN THE OIL, THE PARTIAL NEUTRALIZING BEING COMPLETED WHILE FLOWING THE MIXTURE UNDER CONDITIONS OF MILD TURBULENCE, CENTRIFUGING THE MIXTURE IN A CENTRIFUGAL CHAMBER TO FREE THE OIL FROM SOAP AND ACID SOAPS, AND, IN THE SECOND STAGE, MIXING THE REMAINING OIL FROM THE FIRST STAGE WITH A FURTHER QUANTITY OF STRONG LYE TO NEUTRALIZE RESIDUAL FATTY ACIDS AND ACID SOAPS AND PRECIPITATE PIGMENT AND SLIMY MATTER, THE RECENTRIFUGING SAID REMAINING OIL TO SEPARATE THE RESULTING SOAP, PIGMENT AND SLIMY MATTER THEREFROM. 